The role of dissolved organic material in the nutrition and survival of marine dinoflagellates

Some non-heterotrophic phytoplankters appear to be capable of photoassimilating exogenous dissolved organic carbon and it has been suggested that these phytoplankters may utilize such material as an auxiliary energy source to photosynthesis especially during low light exposure. To test this hypothesis for marine dinoflagellates, the growth and survival of six axenic species were monitored at various light intensities and in darkness in the presence of 10 μM glycerol, 10 μM glucose, and 10 μM acetate. These species were also examined for their ability to incorporate glucose, glycerol, acetate, glycollate, and glycine at approximately natural substrate concentrations. In no case did organic material affect the growth or survival of an axenic species. Bacterized cultures of Amphidinium carterae did exhibit some growth stimulation in the presence of dissolved organic substrates but the same strain when purified no longer revealed this stimulation. At least one species incorporated significant quantities of labelled organic carbon but this incorporation decreased with decreasing light intensity and never exceeded 1.6% of the inorganic carbon fixed at 60 μE/m2/sec. It is concluded that the photoassimilation of organic carbon by autotrophic phytoplankters is not likely to be an important factor in their low light survival.